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A planet is moving in an elliptical orbit around the sun. If T, V, E and L stand respectively for its kinetic energy, gravitational potential energy, total energy and magnitude of angular momentum about the centre of force, which of the following is correct?

1. T is conserved
2. V is always positive
3. E is always negative
4. L is conserved but direction of vector L changes continuously

Correct answer: E is always negative

Solution

For a planet in an elliptical orbit, the total energy (E) is always negative because the gravitational potential energy dominates over the kinetic energy, indicating a bound system. Angular momentum (L) is conserved in both magnitude and direction, so option D is incorrect.

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